Method for controlling the power take-off shaft of a tractor

ABSTRACT

The invention relates to a method for controlling the drive of the power take-off (PTO) shaft of a tractor in which the PTO shaft can be connected to the drive engine by means of a PTO shaft clutch. The tractor comprises a hydraulic power lift to which a working machine driven by the PTO shaft can be coupled. In order to improve safety and operational comfort, the inventive method is comprised of the following steps: a) the PTO shaft clutch is engaged by means of a pulse-width-modulated on/off valve when the switch for turning on the PTO shaft is activated such that the clutch starts working in a controlled manner; b) if, afterwards, the switch for changing over to an automatic mode is actuated, the PTO shaft clutch is disengaged when the power lift is lifted; when the lift is not lifted, the PTO shaft clutch remains engaged: c) if the power lift is lowered again within a predetermined time interval, the PTO shaft clutch automatically reengages; however, the clutch no longer engages after this time interval has elapsed.

FIELD OF THE INVENTION

The invention relates to a process for controlling the drive of atractor power take-off (PTO) shaft wherein the PTO shaft can beconnected to the drive engine by a PTO clutch, and the tractor has ahydraulic lift to which an implement driven by the PTO shaft can beconnected.

BACKGROUND OF THE INVENTION

The PTO shaft is a high-speed, high-torque working shaft. This shaft andthe connected implements represent a considerable hazard to theoperator. If the implement becomes clogged or is obstructed, he mustlift the implement and leave the operator's cabin to remedy the problem.Operating elements are also frequently provided outside the operator'scabin. To increase safety in such situations, it is known in thepractice to automatically disengage the PTO clutch when lifting theimplement.

Differentiated safety devices are considered unsafe because of the riskof malfunctions, although they can provide a desired increase inoperating comfort. For example, when turning at the edge of a lot, theoperator must not only operate the vehicle but also lift the power liftand actuate the PTO clutch. This can produce harsh operating conditions,reducing the service life of the vehicle and implement.

SUMMARY OF THE INVENTION

Therefore, the goal of the invention is to provide a process forautomatic control of a PTO shaft so as to fully maintain safety in allsituations while requiring the least possible technical effort. Inaccordance with the invention, this is achieved by the followingoperating steps:

a) When the switch (11) is actuated to turn on the PTO shaft, the PTOclutch (2) is engaged by a pulse-width-modulated on/off valve (16) toinitiate this as an automatic process;

b) If switch (12) is subsequently actuated for transition to anautomatic mode, the PTO clutch (2) is disengaged when the power lift (7)is raised; if no lifting is done, the PTO clutch (2) remains engaged;

c) If the power lift (7) is lowered within a prescribed length of time,the PTO clutch (2) is automatically engaged again; after this timeelapses, it no longer engages.

Such a control provides for smooth starting after a prescribedpulse-width gradient or rpm gradient and as a function of the rpm'stransmitted to the implement, and because of the pulse-width-modulation,a straightforward inexpensive on/off valve can be used. The automaticoperation is not obligatory. It can be selected as desired as anoperating mode even after operating for some time in the non-automaticmode. In terms of safety engineering, this is taken into account by thefact that, after the automatic mode is selected, the state of the liftis interrogated. If the power lift is raised, the PTO shaft isdisengaged, as with known devices, but without additional effort. Theprescribed time within which the PTO shaft is automatically engagedagain, in the automatic mode, when the lift is lowered is selected sothat just enough time is available for turning the edge of the field,but when the lift is lowered after an interruption in the work or afterthe operator leaves the cabin, the PTO shaft will not automaticallyengage again. Thus, the required safety is guaranteed for the connectionof lift and PTO shaft engagement.

The extra technical effort is minimal because the inventive process isimplemented by software. Consequently, the automatic engagement forlowering the lift after a prescribed time interval can be made to dependon additional safety controls.

In controlled starting, a safety measure can be included: The startingof the PTO shaft is interrupted immediately by disengaging the clutchwhen the switch for actuation is not operated during a certain timeinterval. Observations have shown that faulty operator motions areidentified as they occur but one second late. However, they can beavoided if the operator is obligated to actuate the switch and must doso for a specific length of time.

Preferably, the engagement of the PTO shaft is smooth for a certaininitial time interval, and only after this time interval has elapsed canthe system be switched to automatic mode. A particularly impact-free andconservative start of the PTO shaft is achieved by taking into accountthe momentary rotational speed of the shaft during controlled startingof the PTO shaft. For this purpose, the pulse-width at which the PTOshaft starts to move is taken into account. A starting pulse-width isprescribed and is increased until the PTO shaft is moving appreciably.The subsequent portion is slowly traversed.

In a further refinement of the process, when the PTO shaft is switchedon, after a prescribed second time interval, which is longer than thefirst time interval, interrogation is done to determine whether the PTOshaft is actually turning; if not, the PTO clutch is disengaged again.This not only protects the clutch from overload and excessive wear butalso protects the operator. At low external temperatures, the PTO shaftmay start with a long lag time and the operator may look around to seewhy the PTO shaft is not starting. This is a frequent cause of seriousaccidents.

In a preferred process, after the power lift is raised, interrogation ispreformed to determine whether the tractor is moving or stopped; if itis moving, the clutch is engaged automatically again only when the powerlift is lowered again within a prescribed initial time interval; if itis stopped, the clutch is engaged automatically again only if the powerlift is lowered again within a prescribed second time interval; theprescribed second time interval is shorter than the prescribed firsttime interval. Thus, the time interval is selected. When the tractor hasstopped, it is shorter than the time it takes the operator to climb outof the operator's cabin. During travel, it is long enough for a turningmaneuver at the edge of the field.

If the tractor also has switches for the power lift located outside theoperator's cabin, then in a very safety-related further refinement ofthe process, the PTO clutch is immediately disengaged and the automaticmode switched on as soon as one of the switches outside the operator'scabin is operated. If in the case of a malfunction, the operator stops,gets out and lifts the implement, the implement is absolutely guaranteedto stop (because the automatic mode is switched on). The malfunction canthen be eliminated without risk of an accident. The automatic mode canbe re-activated only when the operator has returned to the operator'scabin.

In a further refinement of the process, before the PTO clutch isengaged, the engine rpms are interrogated, and the engagement process isstarted only when the engine speed is within a specific rpm range. Thisprevents both jolts and stalling of the engine. Finally, the designingof a control with a processor programmed to accomplish the processherein described is within the scope of the invention. Although thecontrol can also be designed with hardware, processor control isparticularly straightforward and readily adaptable to the vehiclerequirements in question.

DESCRIPTION OF THE DRAWINGS

In the following, the invention is described and explained on the basisof figures. The following are shown:

FIG. 1—a tractor with controls, schematic;

FIG. 2—a flow diagram of an operating phase;

FIG. 3—a flow diagram in automatic mode; and

FIGS. 4 and 5—two flow diagrams with additional safety functions.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 the engine is not shown but is denoted by 1. To the engine isconnected a PTO clutch 2, possibly followed by a multi-step reductiongear for selection of the rated speed of the PTO shaft (540, 750, 1,000or 1,400 rpm). The PTO clutch 2 is a hydraulic clutch of any design, forexample, hydraulic or dry, with one or more disks. From there, a PTOshaft 3 leads to the input shaft 4 of an implement 5 driven by it. Therear axle 6 of the tractor is not shown in detail. Also mounted on thetractor is a power lift 7, which is moved by a hydraulic system 8, whichobeys a lifting gear lever 9 in the operator's cabin. In the embodimentillustrated here, the power lift and PTO shaft are arranged at the rearof the tractor; they could also be arranged at the front of the tractor.

A control unit 10 for the PTO clutch is provided at a suitable area inthe tractor. The unit contains a processor which repetitively runs theinventive control program and is connected via lines (only indicatedschematically) with the following operating elements and sensors: aswitch 11, preferably a pushbutton switch, for switching on the PTO; aswitch 12 for switching to automatic mode, preferably with twopositions; a switch 13, a pushbutton, for switching off the PTO; aswitch 13A additionally provided on a control panel 14 accessible fromoutside the vehicle, for switching off the PTO shaft, together with anadditional actuating device 15 for the power lift 7; and on/off valve 16for the PTO clutch, i.e., a pulse-width-modulated on/off valve; a sensor21 for the engine speed; a sensor 22 for the PTO shaft rotational speed;a sensor 23 for the position of the power lift and a sensor 24 for therotational speed of the rear axle, i.e., the speed of the tractor.

FIG. 2 shows the steps involved in switching on and starting the PTOshaft. To initiate the engagement, pushbutton 11 is pressed (Field 30).In accordance with Field 31, interrogation is done to determine whetherpushbutton 11 is still depressed during a first time interval (about 1.5seconds). If pushbutton 11 is released before this first time intervalelapses, the command is not accepted, Field 32. If the pushbutton 11 ispressed long enough, valve 16 for starting the PTO shaft is actuated,Field 33. Within an initial time interval, the start is accomplished bystarting the pulse-width-modulated valve 16 in accordance with a storedstarting function (Field 34). For jolt-free control of the start, thestarting of PTO clutch 2 is influenced by the pulse-width or the pointin time at which the PTO shaft starts to move, which is signaled to thecontrol unit 10 via sensor 22. At this point, the PTO should be running.To confirm this, after a second time interval (about 10 or 20 seconds),another check is made according to Field 35 on the basis of the PTOshaft speed signaled by sensor 22 to determine whether the PTO shaft hasactually started. If not, the PTO clutch 2 is immediately disengagedagain (Field 36). If the control in Field 35 is positive, the startingprocess is stopped, Field 37.

FIG. 3 shows the process sequence in automatic mode, here also called“management.” One can switch to this mode only if the PTO shaft hasalready started to turn. For this purpose, switch 12 (FIG. 1) is placedin the position “management pushbutton on” (Field 40). For safety'ssake, the system interrogates, according to Field 41, whether the powerlift is raised or is being raised. If not, the PTO shaft continues torun normally (Field 42) until, at any subsequent point in time, thepower lift is raised by actuating lift lever 9. This lifting is signaledby sensor 23 to control unit 10. As soon as the signal is received, thePTO clutch 2 is disengaged, thereby switching off the PTO shaft drive(Field 43).

For automatically re-starting the PTO shaft when the power lift islowered, a case distinction is made between two operating states. Forthis purpose, sensor 24 first interrogates. Field 44 whether the tractoris in motion. If it is in motion, a time gate is opened (Field 45), sothat the automatic re-starting of the PTO shaft occurs only when thepower lift is lowered again within the time interval (2 minutes)determined with this time gate. If the power lift is not lowered (Field46), the PTO shaft remains switched off (Field 47). If the power lift islowered, interrogation is done in Field 48 to determine whether thefirst time interval has elapsed. If it has elapsed, both the PTO shaftand the automatic mode are switched off (Field 49). If it has notelapsed, the PTO clutch is engaged again (see FIG. 2, from Field 33 onand Field 50 in FIG. 3). However, if the tractor is stopped, theprescribed second time interval is considerably shorter, about, 10 sec.The first time interval is designed so that, after the tractor turns atthe edge of a field and the power lift is lowered again, the PTO shaftis switched on again. The predetermined second time interval is designedso that it elapses before the operator has the time to leave theoperator's cabin. If the lift is not lowered (Field 52) the PTO remainsdisengaged (Field 47); however, if the lift is lowered after the 10second interval (Field 53), the PTO and automatic mode are shut offindefinitely (Field 49). If, on the other hand, the lift is loweredwithin the 10 second interval (Field 53), the PTO starts once more(Field 50). Thus, it may happen under certain circumstances that theoperator is injured while manipulating the implement.

FIGS. 4 and 5 show another two safety functions. In accordance withFields 60, 61, the PTO shaft is switched off at highest priority wheneither of the “PTO stop” pushbuttons 13, 13A, i.e., either 13 in theoperator's cabin or 13A accessible from the outside, is actuated.

In accordance with Fields 62, 63, the automatic mode is deactivated andhence the PTO shaft switched off simultaneously as soon as one ofactuating elements 13A or 15 of the vehicle's switching unit 14accessible from the outside is actuated, i.e., even if only the powerlift is being actuated. Frequently, the switching unit 14 is designedwithout the actuating element 13A.

Automatic mode can be started only from within the cabin and is alsodeactivated by actuating either pushbutton “PTO stop”13 or 13A. Thenautomatic mode can be switched on again only if the PTO shaft is startedagain. Thanks to the present invention, absolute safety is combined witha maximum of operator comfort in the manner described herein.

What is claimed is:
 1. A method for controlling a drive of a tractorpower take-off (PTO) shaft, wherein the PTO shaft can be connected to adrive engine via a PTO clutch, and a tractor possesses a hydraulic powerlift to which an implement driven by the PTO shaft can be connected,comprising the following steps: a) actuating a first switch forswitching on the PTO shaft, whereby the PTO clutch is engaged by apulse-width-modulated on/off valve, so as to start in a controlledmanner; b) actuating a second switch to an automatic mode, whereby thePTO clutch is disengaged when the power lift is raised; if the lift isnot raised, the PTO clutch remains engaged; and c) sensing if the powerlift is lowered within a prescribed time interval, whereby the PTOclutch is automatically engaged again, but after this prescribed timeinterval expires, the PTO clutch does not engage again.
 2. The methodaccording to claim 1, further comprising the step of sensing when thePTO shaft is switched on, the starting of the PTO shaft is immediatelyinterrupted by disengaging the clutch if the first switch does notremain actuated for a first time interval.
 3. The method according toclaim 2, further comprising the step of sensing when the PTO shaft hasbeen switched on for a specific initial time interval, and only afterthe initial time interval has elapsed can the automatic mode be switchedon.
 4. The method according to claim 3, further comprising the step ofinterrogating to determine when the PTO shaft has been switched on for asecond time interval, which is longer than the first time interval,whether the PTO shaft is actually turning; if not, the PTO clutch isimmediately completely disengaged.
 5. The method according to claim 1,further comprising the step of sensing a momentary speed of the PTOshaft when said shaft is started in a controlled manner.
 6. The methodaccording to claim 5, further comprising the step of sensing the pulsewidth at which the PTO shaft begins to move.
 7. The method according toclaim 1, further comprising the step of interrogating after the powerlift is raised to determine whether the tractor is moving or stopped; ifit is moving, the clutch is automatically engaged only if the power liftis lowered again within a first time interval; if it is stopped, theclutch is engaged automatically again only if the power lift is loweredagain within a second time interval; the second time interval is shorterthan the first time interval.
 8. The method according to claim 1,wherein the tractor additionally possesses control switches locatedoutside an operator's cabin for the PTO shaft and the power lift,characterized by the fact that the PTO clutch is immediately disengagedand the automatic mode switched off as soon as one of the controlswitches outside the operator's cabin is actuated.
 9. The methodaccording to claim 1, further comprising the step of interrogating thedrive engine speed and engaging the PTO clutch only if the engine speedis within a certain range.